During recent years there has been an extensive effort to lower the amount of hydrocarbon and carbon monoxide emitted to the atmosphere by internal combustion engines. Substantial reductions have been achieved by such means as using lean air/fuel mixtures, injecting air into hot exhaust, replacing the exhaust manifold with a thermal reactor or placing a catalytic reactor in the exhaust system. Use of an exhaust catalyst is generally more effective if the engine is operated on lead-free or low-lead (i.e., tetraethyllead) gasoline. Thermal exhaust reactors are effective no matter what type of gasoline is used. Thus, there are substantial economic benefits which are achieved by use of a thermal reactor rather than a catalytic reactor.
Thermal reactors in which the hot exhaust passes from the exhaust port into an enlarged, usually insulated chamber are quite effective during normal modes of engine operation. These are generally used in conjunction with air injection using lean air/fuel mixtures. One problem encountered with such lean systems is that during idle the amount of carbon monoxide in the exhaust is too low to provide sufficient combustion heat to keep the thermal reactor and related hardware at a sufficiently high temperature. Furthermore, if unrestricted air injection is used during all modes of engine operation the amount injected at idle can be so excessive that the air cools the exhaust mixture below combustion temperature thus allowing the exhaust passage and thermal reactor to cool. The result of this is that when the engine shifts into an operating work load the thermal reactor is initially ineffective because of its temperature which permits increased amounts of hydrocarbon and carbon monoxide to be exhausted.